Actomyosin ATPase activities in the brachiopod Terebratalia transversa

Abstract

Actomyosin specific adenosine triphosphate (AM-ATPase) activities of muscles of the articulate brachiopod Terebratalia transversa were determined in an attempt to understand some of their unusual physiological properties. The striated adductors, smooth adductors, and ventral adjustors have activities of 0.318, 0.089, and 0.050 μmol of inorganic phosphate per minute per milligram of protein respectively, which correlate with their contraction rates and behaviors. The suggestion that sudden compliance to stretch in the diductor muscles (slip) is due to simultaneous breakage of all cross bridges is supported by the low activity of 0.022 μmol of inorganic phosphate per minute per milligram of protein. This rate of activity is not as low as expected from the extremely slow rate of isometric contraction (186 min) but it does correlate with the faster rate of isotonic contraction observed after removal of the antagonistic smooth adductors. Slippage represents a previously unknown extreme in the macromolecular organization and kinetics of a contractile system, an elegant adaptation of articulate brachiopods to minimize the metabolic cost of keeping the valves open while allowing rapid closure in an emergency.